Marriage or carnal commerce between persons of a close degree of consanguinity. Even in modern times the connotation of "incestuous" is not the same in all countries. Among primitive and barbarous races there is a still wider divergence. Nor has the opinion as to which marriages between relatives were incestuous and hence forbidden been constant at all times among the Israelites. The oldest customs were laxer in permitting marriages than was the law of the intermediary books of the Pentateuch. The marriage of the father with his own daughter (and therefore presumably also that of the son with his mother) was forbidden at all times as incestuous. The story of Lot, which might be construed as showing that even this relationship was allowed in Ammon and Moab (Gen. xix. 30 et seq.), reflects the antipathy of Israel, which regarded these peoples as born of an incestuous union. But of other marriages forbidden in olden times as incestuous no definite data are obtainable. Endogamic marriages (i.e., within the circle of one's relatives) were preferred by ancient tribes. The chosen suitor for a girl was her cousin; it was actually forbidden for the eldest daughter to marry outside the family. By analogy, then, the conclusion is safe that marriages between very near relatives were permitted among the ancient Hebrews also. In fact, there is no lack of evidence for this. Abraham, whose wife Sarah was also his half-sister, may be mentioned as an example of a marriage between brother and sister (Gen. xx. 12). Even in David's time, although it is represented as unusual for a royal prince to marry his sister (II Sam. xiii. 13), it was still regarded as neither objectionable nor forbidden. It should be noticed that in both these cases the union was with a paternal half-sister; the husband and wife being of one father, but not of one mother. Jacob had to wife two sisters at the same time, and Moses was born of a marriage between nephew and aunt (Num. xxvi. 59). Marriage with a sister-in-law, or the widow of a deceased brother, is in certain cases a religious duty (see Levirate): only from the account of Judah and Tamar (Gen. xxxviii.; comp. especially v. 26) is it to be concluded that in case of a lack of brothers the oldest custom obliged the father to marry his daughter-in-law.

It has been contended that marriage with the father's wife (who was not the son's own mother) seems not to have been objectionable in olden times. As an instance of this the union between Reuben and Bilhah is adduced (Gen. xxxv. 22). But in Gen. xlix. 4 this union is severely condemned. The right explanation of this incident as well as of the similar occurrence reported in the story of Absalom's uprising (II Sam. xvi. 21, 22) is that control of the harem of one's predecessor was regarded as the assertion of one's right to the throne. And when Adonijah asks for Abishag from his father's harem, he appears from this act to claim to be his heir (I Kings ii. 13 et seq.). The phrase (Gen. xlix. 4) may be taken symbolically, and doesnot necessarily convey the idea of an actual incestuous union. The following, however, are the degrees of consanguinity and relationship within which marriage is forbidden as incestuous in Deuteronomy: the father's wife (xxi. 30, xxvii. 20); a sister or half-sister (xxvii. 22); and a mother-in-law (xxvii. 23). In all three points, however, even in Ezekiel's time, custom by no means upheld the law (Ezek. xxii. 10 et seq.).

The so-called Priestly Code goes furthest in forbidding marriages among relatives. According to Lev. xviii. 6-18, a man may under no circumstances marry: (1) mother, (2) stepmother, (3) sister, (4) son's daughter, (5) daughter's daughter, (6) half-sister, father's side [or mother's side], (7) father's sister, (8) mother's sister [aunt], (9) wife of father's brother, (10) daughter-in-law, (11) sister-in-law, (12) wife and her daughter [or wife and (16) her mother], (13) wife's son's daughter, (14) wife's daughter's daughter, or (15) wife and her sister [both living]. In Lev. xx. 11-21 another list is given, which enumerates only Nos. 2, 6, 7, 8, 9, 10, and 12, and omits those that are implied, such as mother's sister, granddaughter, and sister-in-law; explaining also that No. 6 includes a half-sister on the mother's side, and that No. 12 includes wife and her mother. This chapter describes the punishments of the various classes of incest (see Punishment).

The same unions were in general forbidden by Islam, as also by custom earlier than Islam.

The crime of incest is known in the Talmud as "'arayot"; and it is implied that alliances involving its commission are illegitimate and consequently null and void.

A notable omission from the list of those with whom sexual intercourse, according to Lev. xviii., constitutes incest is a daughter, in regard to whom the prohibition is explained by the Talmud as "self-evident" or implied from the expressed proscription against a granddaughter (Yeb. 3a). Deut. xxvii. 20, 22, 23, as was noted above, enumerates only Nos. 2, 6, and 12, namely, father's wife, half-sister, and mother-in-law; this, according to the Rabbis, is because they are more remote [the others being implied], and because, since they usually live together in the same house, if they violate the law they can not be easily detected (Rashbam, Commentary). The intercourse of such relatives is among the "secret sins" to which the Levites' curse on Mt. Ebel was directed (Deut. xxvii. 15). The levirate marriage of the childless wife of a dead brother (), though commended in the Bible, is discouraged by some rabbis. Abba Saul said that "ḥali-ẓah" is preferable to marriage (Yeb. 3a). Later it was prohibited in European countries. See Levirate Marriage.

The soferim or scribes (322-221 B.C.) extended the number of degrees of relationship within which marriage involved incest, and ranked those relations as "seconds" () or subordinates which are not included in the Bible. Marriage with these was forbidden by the Rabbis () as a precaution and safeguard against the infringement of the Mosaic degrees (; Yeb. 21a). The rabbinical "seconds" are as follows: [19] mother's mother; [20] father's mother; [21] wife of father's father; [22] wife of mother's father; [23] wife of father's brother, on the mother's side; [24] wife of mother's brother, on the father's side; [25] son's daughter-in-law; [26] daughter's daughter-in-law (Tosef., Yeb. ii. 3). The prohibition is thus raised one degree on the ascent, and one degree on the descent in the case of the daughter-in-law; while the prohibition of the wife of a father's half-brother is balanced by the prohibition of the wife of a mother's half-brother on the mother's side (being comparative seconds to the Mosaic half-sister prohibition).

R. Ḥiyya, in his list of seconds, or rather "thirds," goes one step further, and adds the third generation on the descent, namely: [27] daughter's granddaughter, and [28] son's granddaughter; likewise a wife's third generation [29] and [30]. On the ascent he includes the fourth generation and prohibits the grandmother of a wife's mother or father [31] and [32] (Yeb. 22a). A like prohibition on the man's side is implied, but not mentioned, the existence of relatives of this degree being an improbability, except on the wife's side, who usually was the husband's junior. It is questionable whether R. Ḥiyya's seconds are infinite, i.e., whether the prohibition is endless, both on the ascent and the descent, or whether it stops at the point described (ib.). Rab is of the opinion that the prohibition stops with the wife of a mother's brother [24], and goes no further, even on the father's side; nor above the wife of a father's brother on the mother's side [23]; nor below a daughter's daughter-in-law [26]. Ze'era permits the wife of the father of a mother [22] (ib. 21a). Rab denies this permission, as it might be mistaken to refer to the wife of a father's father, whereas she, as well as the wife of any of a father's direct ancestors, to the infinite degree, is prohibited. Ze'era, however, thought there was no chance for an error, as a man is not in the habit of visiting his mother's family in like manner as his father's (ib.). Beyond the line of seconds, affinitive incest, according to Rab, stops, but consanguinitive incest is infinite. Accordingly the marriage of any of the direct descendants of Abraham with any of those of Sarah, to the end of humanity, would be prohibited (Yer. Yeb. ii. 4).

Bar Ḳappara adds to the seconds the mother of the father of one's mother [33], and the mother of the father of one's father [34], and thinks that incest stops both above and below the line of seconds. R. Ḥanina, however, is of the opinion that the seconds which are specifically mentioned include merely those with whom the natural length of human life allows marriage to be thought of as a probable contingency; but the prohibition extends to infinity, except in the case of a mother's father's wife (ib.).

Principal and Second Degrees of Incest, of Kindred and Affinity, in Ascending and Descending Scales, Prohibited to the Man.

Rab rules as a second a male whose female prototype is prohibited in the Mosaic law, and thus includes among the seconds the wife of a father's or mother's brother [23] and [24]; also his son's or daughter's daughter-in-law [25] and [26] (ib.); but he excepts the wife of a father-in-law (40) and the wife of the son of a mother-in-law or father-in-law, or the wife of the son of a stepson; these are permitted, for the reason that in these cases the affinity is not direct, but requires two distinct marriages to bind the kinship (Yeb. 21b). There is no incest between one's wife and his stepson, nor between his stepson and his daughter, although a stepdaughter is prohibited in the Bible (Tosef., Yeb. ii. 3); nor between two stepchildren, that is, one his own and the other his wife's, who may intermarry, though they both live in the same house. R. Eleazar, however, prohibited their marriage for appearance's sake, and R. Ḥanina would permit it only in a place where the parties are unknown as stepchildren (Yer. Yeb. ii. 4). Amemar permitted the wife of the brother of a father's father (37), and the sister of a father's father (38); while other authorities prohibit them (Yeb. 21b). The authorities agree on the prohibition of the son's son's daughter-in-law infinitely, on the ground that the inheritance line is continuous on the son's side, and because father and son usually visit each other, whereas on the daughter's side both the inheritance and the visits cease (Tosafot, s.v.; Yeb. 22a).

The principal reason for prohibiting the great-grandmother, though she is not on the inheritance line, is because she is likewise called "grandmother" (). A similar reason is applied to the great-granddaughter. R. Ḥana derives the prohibition against the third generation, both ascending and descending, from the specific proscription against the wife's grandchild in Lev. xviii. 17 (Yer. Yeb. ii. 4). Some authorities prohibit the grandmother's sister (39) and also the marriage of a man to the wife of the former husband of his wife (41) ("Tif'eret Yisrael" to Yeb. ii. 1).

David took Rizpah, the wife of his father-in-law Saul (II Sam. xii. 8), which is permitted according to the Biblical law, though R. Ḥanina prohibits a wife's stepmother for appearance's sake (Yer. l.c.). But the Talmud Babli permits a father-in-law's wife. The Babylonian Talmud is less strict in regard to the degree of relationship which renders a marriage incestuous than the Jerusalem Talmud, a difference which furthermore divides the Sephardim from the Ashkenazim ("Bet Yosef" to Ṭur Eben ha-'Ezer, xv. 39a). The former, led by Maimonides, are guided by the Babylonian Talmud, while the Ashkenazim, headed by Asheri and Caro, concur with the Jerusalem Talmud.

The later authorities in Europe were even more rigid, as the condition of their countries and the development of the time warranted a stricter observance of the law against incest. Thus Rabbenu Tam in France stopped the marriage of a man to the wife of his father-in-law, and spoiled the banquet and all preparations for the wedding (ib.). Yet the Sephardim permit such a marriage. In a case presented to Rabbi Nathanson he rules to prohibit it (Responsa, "Sho'el u-Meshib," iii., No. 29), and where the marriage has already taken place would compel the husband to divorce his wife; making an exception, however, if she has borne him children, so as not to reflect on their legitimacy. The responsum is dated 1857.

There is a difference between Maimonides, who is against, and Asheri and Caro, who are for, the infinite extension of the prohibition beyond the line of seconds of the wife's ancestors and descendants to the third generation, also below the third generation on the man's side, except the daughter-in-law from son to son. But all authorities agree that the man's parental line is infinite except in cases indicated.

The majority of the rabbis permit the illegitimate (seduced) wife of a father or of his son. R. Judah prohibits the former (Yeb. 4a). But the decision is against him, though there is no question as to the prohibition of an illegitimate daughter or granddaughter. Cousins german are permitted to marry, and to marry the daughter of a sister (a niece) is even advised as a meritorious act (Yeb. 62a, and Rashi).

The difference between the principal (Biblical) degrees of incest and the rabbinical seconds is that the marriages involving the former are considered illegal, requiring no divorce, and the issue is declared illegitimate, while the marriages involving the latter must be dissolved by a divorce, and the children are legitimate (Shulḥan 'Aruk, Eben ha-'Ezer, 16, 1). Incest by affinity is disregarded when the first marriage is not legal (Yeb. 94b).

Prior to the enactment of the Mosaic law on Sinai, a Noachid was prohibited only the natural degrees of incest, such as were later capitally punished by the Jews (Sanh. 57b). Maimonides enumerates them as follows: marriage with (1) mother, (2) father's wife, (3) married woman, and (4) sister on the mother's side ("Yad," Melakim, ix.). Hence Abraham was permitted to marry his half-sister on the father's side, and Jacob might marry two sisters because these cases were not contrary to the natural law, although they were later prohibited by the laws of Moses. It should be noted that the Noachian law was more rigorous on the mother's side and the Mosaic law stricter on the father's side, as the former was based on nature and the latter on the civil law of inheritance and social connections.

Special rules were made for teaching the laws of incest: "Whoever puts a different interpretation upon 'arayot at the public reading of the Pentateuch shall be stopped" (Meg. vi. 9). The teacher must explain the various grades of incest to each student separately; therefore "'arayot shall not be taught in public" (Ḥag. ii. 1), as one might be inattentive and misinterpret the Law. The chapter on incest (Lev. xviii.) is read on the most solemn day, Yom Kippur, to impress the public with its importance.

[Reference-numbers in parentheses in the article Incest correspond with names of relatives printed in capitals in table; those in brackets with the names in small letters; those in italics with the names in italics.]

Among the points on which Karaites and Rabbinites were divided was the interpretation of the Biblical laws concerning incest. Applying to these laws the hermeneutic rule of analogy ("heḳesh"), Anan, the founder of Karaism, was more strict than the Rabbinites, who laid down the principle that the laws concerning incest were not subject to the hermeneutic rules of interpretation. Anan's immediate successors went still further. Assuming the principle that husband and wife are to be considered legally as one person, the Karaite expoundersof the Law, known as "ba'ale ha-rikkub," prohibited the marriage of the husband to the wife's relatives, regarding them as being related to him in the same degree as they are to her. On the same principle, the prohibition was extended to the relatives of the second, third, or fourth husband of a divorced wife. A stepsister, because of the name "sister," was classed by them as a sister, the prohibition being made to apply to her relatives as well as to those of a real sister. The Biblical prohibition of Lev. xviii. 17 applies, according to them, not only to a wife's direct daughter, but also to her stepdaughter, and even to her husband's stepdaughter.

In the eleventh century two expounders of the Law, Joseph ha-Ro'eh and his pupil Jeshua, started a reform movement. They refuted the arguments upon which the ba'ale ha-rikkub based their principle that husband and wife are to be considered as one person, and rejected their prohibitions based on "appellation," e.g., the prohibition against marrying a stepsister on account of the name, and the prohibition derived "by inversion," as that of marrying a woman and her stepdaughter. Only the prohibitions enumerated in the Pentateuch and those derived from them by the application of the hermeneutic rule of analogy were recognized by Joseph ha-Ro'eh and Jeshua, whose views were ultimately adopted by all Karaites.

These prohibitions, both expressed and derived, are divided into five categories according to Joseph, into six according to Jeshua. To the first category belong those referring to the six relatives known in legislation as (= "issue of flesh"), namely, mother, stepmother, sister, sister-in-law, daughter, and daughter-in-law. Of these prohibitions, five are expressed and one (that of the daughter) is derived. According to Jeshua, the prohibition in this category is infinite, both in the ascending line (e.g., grandmother, great-grandmother, etc.), and in the descending (e.g., granddaughter, great-granddaughter, etc.). The second category comprises the prohibitions of relatives in the second degree (), namely, aunt (father's side or mother's side, by blood or by alliance), granddaughter (by son or daughter), and son's or daughter's daughter-in-law. The prohibition in this category is infinite in the direct line, but stops at the point described in the collateral line. To the third category belong the prohibitions against marrying two women who are related in the first degree, as mother and daughter, sisters, sisters-in-law, a mother and her daughter-in-law.

By analogy the prohibition is extended to the "rivals" of the prohibited women, as the wife of the mother's, sister's, and sister-in-law's husband. The fourth category prohibits marrying two women who are reláted in the second degree, namely, grandmother and granddaughter (by the son or by the daughter), aunt and niece (father's side or mother's side), grandmother-in-law and granddaughter-in-law (by the son or by the daughter).

The fifth category prohibits the marriage of parallel related pairs, as of a father and son respectively to a mother and daughter, or to two sisters; of two brothers to mother and daughter, or to two sisters or two sisters-in-law; the prohibition affecting both the ascending and the descending lines, the direct and the collateral lines. Stepbrothers are considered as brothers, and the prohibition contained in this category is applied also to them.

The sixth category prohibits marrying a woman one of whose relatives in the first degree, as, for instance, her mother, or her daughter, has married one's relative in the second degree, as, for instance, a grandfather, grandson, or uncle. Jeshua infers from the omission of the word (= "kins-woman") in Lev. xviii. 14 that "brother" includes the stepbrother, to whom the prohibition contained in the sixth category is extended.